1. Field of the Invention
This invention relates to a method of forming a layer of, for example, the carbide or nitride of chromium or vanadium on the surface of the material to be treated, such as an article made of an alloy of iron, in a fluidized bed furnace, and an apparatus which is used for carrying out the method.
2. Description of the Related Art
There are known methods which employ a fluidized bed to form a carbide or nitride layer on the surface of the material to be treated, as described in Japanese Patent Application No. 213749/1982 (Laid-Open No. 107990/1984) and Japanese Patent Application No. 108054/1984 (Laid-Open No. 251274/1985, U.S. Pat. No. 4,569,862) which cover prior inventions of the inventors of this invention.
These methods employ a treating agent which comprises a fluidized bed-forming refractory powder, such as a powder of alumina; a powder of a layer-forming agent selected from a metal element for forming a carbide or nitride and an alloy thereof; and a powder of an activator selected from an ammonium halide and a metal halide which is sublimable or vaporizable at or below the temperature which is employed for the treatment. The treating agent is fluidized by a fluidizing gas, such as argon, to form a fluidized bed, and the material to be treated is disposed in the fluidized bed, whereby a carbide or nitride is formed on the surface of the material to be treated. A gas of the activator reacts with the layer-forming agent to form a gas of the halide of the element that it consists of, or contains. The halide gas forms a carbide by reacting with carbon in the material to be treated, or a nitride by reacting with nitrogen in the material to be treated (steel), or nitrogen gas which is introduced into the fluidized bed.
It is necessary to employ as the activator a substance which is sublimable or vaporizable at or below the temperature which is employed for the treatment, so that the treating agent may not solidify during its use for the treatment, but may maintain its fluidized state. If the treating agent is used for a long time, its power of forming a surface layer is gradually reduced as a result of a gradual loss of the activator. This is most likely to result in the formation of only a carbide or nitride layer having a smaller thickness with the lapse of time.
A solution to these problems is proposed in Japanese Patent Application No. 159440/1986 (U.S. Pat. No. 4,844,949), which covers another prior invention of the inventors of this invention. According to this prior invention, the treating agent which comprises a fluidizing bed-forming powder and a powder of a layer-forming agent is fluidized to form a fluidized bed, the material to be treated is disposed in the fluidized bed, and the activator is supplied from outside into the furnace, whereby a carbide or the like is formed on the surface of the material to be treated. This method in which the activator is supplied from outside, makes it possible to maintain the layer-forming ability and to carry out an efficient continuous treatment. However, since this method uses the layer-forming agent in powder form, such agent when fluidized is likely to adhere to the surface of the material to be treated, resulting in the degradation of the surface smoothness.